With the vast proliferation of electronic devices of increasing complexity, there is a continual effort to augment the power conversion circuitry. Two of the main types of power conversion circuitry are power amplifiers and power converters. Power amplifiers are widely used in telecommunication and industrial applications and have found use as the front-end stage in high performance dc-dc converters. Depending on their mode of operation, power amplifiers can be classified in two families, namely linear power amplifiers and switched mode amplifiers. Linear power amplifiers are commonly used because of their simplicity, linearity, and good dynamic performance. They are designed with active gain device, usually a transistor, operating in the “linear region” a condition that results in significant quiescent power dissipation. The relatively poor efficiency of linear power amplifiers makes them better suited for applications where linearity is important and the ensuing power losses are manageable.
Switched-mode amplifiers operate with the transistor either fully “on” or “off”, using the transistor effectively as a switch. Switched-mode power amplifiers are relatively efficient and find use in applications where higher efficiency is critical to meet power density, power consumption, or size requirements.
It is generally understood that the difference between an amplifier and an inverter is that an amplifier has a port for an input signal, which in this discussion is an AC input, and converters energy from a DC power source into an AC output. An inverter is simply an amplifier with a self-contained AC signal source to be amplified and delivered to the output. Finally, if the AC output of an inverter is rectified, i.e. converted back to DC, the complete system functions as a DC-DC converter. For illustrative purposes, the description herein basically applies to the cases of amplifiers, inverters, and when rectifiers are included, DC-DC converters.
The vast majority of dc-dc power conversion circuits utilise inductors and capacitors in conjunction with switches to efficiently process electrical power. It is known that higher switching frequency reduces the size and value of the passive components. However, such high frequency operation tends to decrease the efficiency, and there are various soft switching techniques that have been developed to reduce the losses associated with the switching. The soft switching converters for the dc-dc power conversion application typically have an inverter section that converts the ac power that is then processed to generate the required dc power, typically by a rectifier section. In addition to dc-dc converters, other high frequency applications employing resonant inverters include radio frequency (RF) power amplifiers for usage in such fields as healthcare technology and communications.
An operational principle of efficient power conversion is the periodic controlled storage and release of energy, wherein the average flow of power from one port to another is regulated. In principle, power processing thus accomplished is lossless, and in practice, low losses can be achieved. One of the main contributors to the volume of a power processing circuit is the required energy storage, wherein the storage is typically implemented with capacitors and inductors. For a given energy storage technology, the size of the energy storage elements is usually a monotonic increasing function to the energy to be stored. Thus, increases in power density require either reduction in energy stored or increases in energy storage density. The latter is heavily dependent on physics and material science, and furthermore appears to be subject to fundamental limitations such as breakdown voltage and permittivity for capacitors, and saturation flux density and permeability for inductors. Improvement in the material properties of magnetic and dielectric components is a relatively slow process. The alternative is to reduce the required amount of stored energy per operating cycle. For a circuit processing a specified amount of power, this is accomplished by increasing the switching frequency.
Up to a point, increased switching frequency yields increases power density, however as switching frequency continues to increase, issues arise which detract from these gains. These issues include increased switching losses, proximity losses and core losses in magnetic components, and problems with parasitic reactances. While these can be mitigated to some extent, the inefficiency issues tend to dominate the converter design, and further increases in switching frequency increase cost and losses with no attendant increase, or even a decrease, in power density.
In the HF and VHF range, which is defined as being in a range from a few MHz to a few hundred MHz, inverters and rectifiers typically employ soft switching for both turn-on and turn-off, so that switching losses are kept at acceptable levels. The most common inverter topologies used in the HF or VHF band are either based on class D, E, or DE topologies. According to the conventional definition, Class D does not guarantee soft-switching on all transitions, while Class E and DE topologies are distinguished by the use of resonant waveforms and switch transition timing such that all switching transitions are soft, and that any anti-parallel diodes of switches do not conduct. The latter means that reverse recovery losses may be neglected. In Class DE, peak voltage stresses on the switches are advantageously limited to the bus voltage, but driving a high-side switch with the precise timing required becomes difficult as the frequency increases beyond 10 MHz-20 MHz. This is primarily due to common-mode currents in the high-side gate drive. Class E avoids this condition via the use of a single-ended ground-referenced switch, but the trade-off is high device voltage stress. Furthermore, class E and DE inverters are characterized by a relationship between active switch capacitance, switching frequency, and processed power. This relationship severely constrains the practical design space for these inverters, thereby limiting their application.
For switched-mode power amplifiers to operate at frequencies in the HF, VHF, or higher frequency ranges, resonant elements are typically used to bring the switch voltage close to zero right before the switching transition. This condition is normally called Zero-Voltage-Switching (ZVS). ZVS transitions effectively remove the energy that otherwise would be wasted in the semiconductor every switching cycle. A further reduction in switching losses can be achieved by delaying the voltage rise on the transistor as the latter is turned off, thus preventing substantial voltage and current from being impressed in the transistor simultaneously. This is usually accomplished via capacitance across the switch terminals, which generally included the inherent switch capacitance present in all practical switches. It is this same capacitance that necessitates the use of ZVS as described above.
A drawback of many switched-mode resonant inverters is the large voltage (or current) the transistor has to withstand as a result of the resonant process. For example, the Class-E inverter is a well-known switched-mode power amplifier that imposes a high peak voltage across the transistor. Specifically, the transistor sees a voltage reaching nearly four times the input voltage for the standard design.
The class Φ2 inverters are soft switching inverters adapted for high switching frequencies. Also known as Class EF2 inverters, they operate by allowing control of the fundamental switching frequency and the second and third harmonics. They share the soft-switched behavior and the ground-referenced transistor of Class E designs, but have greatly reduced voltage stress and additional design freedom. This allows the class EF2 inverter to have an enlarged design space, and in particular, allows a class EF2 inverter of given input and output current and voltage and a given transistor technology to operate at higher frequency than class E, thus reducing the passive component size.
FIG. 1 is a prior art illustration that shows a class EF2, or equivalently a class EF2 inverter stage 10 as a switched-mode resonant inverter employing a switch Q1, a passive multi-resonant network comprising L1, C1, L2, C2, and a load network L3, C3, and a load impedance ZLD, SE. The switch Q1 is turned ‘on’ and ‘off’ on a periodic basis with switching frequency FS via switch drive signal vD(t). The combination of the multi-resonant network, the load network, and the load impedance creates the impedance ZQ1 as seen by the switch Q1. The exact characteristic of ZQ1 required for proper inverter operation is known, but a useful description is that the impedances at FS and its harmonics are defined to provide both the desired power to the load, and to provide soft switching of the switch Q1. The soft switching behavior allows efficient operation at very high switching frequencies, much higher that practical with standard pulse width modulation (PWM) switching conversion techniques. It is also noted that under periodic steady-state conditions, the values of ZQ1 at FS and its harmonics are the only ones of significance for operation.
Parasitic components often limit the performance of conventional designs operating at high frequencies. For example, the parasitic output capacitance of the transistor Q1 typically limits the maximum frequency at which a conventional class E inverter will operate for a given output power. The EF2 inverter of FIG. 1 overcomes this by providing an extra degree of design freedom, which in turn allows a greater value of transistor capacitance and thus extends the upper end of the operating frequency. Note that the capacitor C1 in FIG. 1 includes this transistor parasitic output capacitance.
A salient characteristic of class EF-type converters is that the voltage across the transistor, vQ1(t), during the off-state is determined by the impedance values of ZQ1(f) at the fundamental, second and third harmonic of the switching frequency FS. Specifically, the low impedance value needed at the second harmonic of the switching frequency is obtained by the addition of a series resonant trap formed by L2 and C2 in FIG. 1. Components L1 and C1 play a major role in setting the impedance ZQ1(f) at the fundamental and third harmonic, and C1 also contributes a decrease in ZQ1 for higher harmonics. This ensures zero voltage switching (ZVS) conditions and helps absorb the transistor capacitance, which is included in C1. The impedance ZQ1 plays a role in the wave shaping of the voltage vq1(t), as well as controlling the power flow from the DC input to the AC output.
The waveshaping reduces the voltage stress across the transistor on the order of 40% as compared to the Class-E. Reducing the peak voltage allows the use of lower voltage semiconductors with better conduction characteristics, and this reduces losses in the inverter, thereby increasing efficiency and allowing higher power density.
Referring to FIG. 2, the simulated impedance ZQ1(f) across the transistor Q1 of the EF2 inverter during the off state of the converter is shown for gain 50 and phase 60. The fundamental switching frequency FS in this example is 30 MHz. The low impedance at the second harmonic (60 MHz) is easily seen. Note that for periodic steady-state operation, only the impedance at multiples of the switching frequency FS are of importance.
The complex impedance ZQ1(f) seen at the fundamental switching frequency f=FS and its second and third harmonics (f=2FS and f=3FS) are important to the operation of the EF2 inverter. The value at f=FS sets both the power level and the ZVS behavior. The values at f=2FS and f=3FS set the shape of the transistor waveform and thus are essential to reducing the transistor voltage stress. In particular, the low impedance at the second harmonic and the relative relationship between the values at fundamental and third harmonic are necessary to produce the quasi-trapezoidal drain waveform characteristic of proper operation.
The complex impedance ZQ1(f) is produced by the network L1, C1, L2, C2, L3, C3, and the load impedance ZLD,SE. The component values must be adjusted, or “tuned” to get the desired impedance values. The difficulty arises in that some of the component values are not adjustable or are non-linear, e.g. the portion of C1 that represents transistor Q1 output capacitance. Therefore, an exact prediction of the required values is not possible. Once initial values are chosen, the component values are tuned either in hardware, in simulation, or in both, in order to get proper operation. This becomes quite difficult because any single component value affects the value of the complex impedance ZQ1(f) at the fundamental switching frequency f=FS and its second and third harmonics (f=2FS and f=3FS). Thus, adjusting one component can require that all other components are adjusted, and so forth. This typically requires a great many iterations to come to an acceptable solution, and is both difficult and time-consuming.
Comparing to the class E, the EF2 inverter achieves a significant reduction in semiconductor voltage at the expense of more resonant components one of which (capacitor C2 in FIG. 1) is rated to almost three times the input voltage.
In dc-dc converter applications, a suitable rectifier capable of operation at high frequency replaces the load. Among the resonant rectifiers topologies commonly used in these applications is the single-diode topology shown in FIG. 3. This single diode rectifier is designed to look resistive at the fundamental frequency, with an input impedance ZLD, SE, so that it may be substituted for the load in FIG. 1. This condition is achieved by resonating the parasitic diode capacitance with a resonant inductor, which also provides the required dc-path for the DC output current.
When this rectifier is connected to the inverter of FIG. 1 in place of the load ZLD,SE, the resonant rectifier sees a mostly sinusoidal input current. The voltage vld,se(t) has a significant harmonic content, but the fundamental component of such voltage is in phase with the current supplied by the inverter. This condition is desirable, but maintained over a narrow output power operating condition.
FIG. 4 shows the simulated input voltage vld,se(t) 400 of the properly tuned resonant rectifier of FIG. 3 and its fundamental component 410.
Referring to FIG. 5, the graph shows the simulated rectifier voltage fundamental component 500 and the input current 510. These waveforms 500, 510 are both are in phase, thereby implying a resistive behavior.
As known in the art, inverters are circuits that convert dc to ac. Note that the same circuits can also be used as amplifiers if the control signal operating the active switch is considered to be an ac input to be amplified. Also note that by adding a resonant rectifier to the output of the inverter, the inverter ac output is converted to a dc output, thereby causing the entire system to operate as a dc-dc converter. Thus, the inverters have great flexibility and depending upon the design criteria such devices are utilised in many different applications.
As noted, electronic devices generally require some type of power conversion in order to operate and there is always a need for greater efficiency and control of the power conversion. Furthermore, there is a continual objective of providing greater functionality in a smaller form factor and the power conversion techniques are desired. Various efforts have been used to improve upon the deficiencies in the conventional designs augment the design capabilities and increase efficiency.